Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Touch screen input devices or sensors for computers and other consumer electronics devices such as mobile phones, personal digital assistants (PDAs) and hand-held games are highly desirable due to their extreme ease of use. In the past, a variety of approaches have been used to provide touch screen input devices. The most common approach uses a flexible resistive overlay, although the overlay is easily damaged, can cause glare problems, and tends to dim the underlying screen, requiring excess power usage to compensate for such dimming. Resistive devices can also be sensitive to humidity, and the cost of the resistive overlay scales as the square of the perimeter. Another approach is the capacitive touch screen, which also requires an overlay. In this case the overlay is generally more durable, but the glare and dimming problems remain.
In yet another common approach, a matrix of infrared light beams is established in front of a display, with a touch detected by the interruption of one or more of the beams. Such “optical” touch screens have long been known (U.S. Pat. Nos. 3,478,220; 3,673,327), with the beams generated by arrays of optical sources such as light emitting diodes (LEDs) and detected by corresponding arrays of detectors (such as phototransistors) They have the advantage of being overlay-free and can function in a variety of ambient light conditions (U.S. Pat. No. 4,988,983), but have a significant cost problem in that they require a large number of source and detector components, as well as supporting electronics. Since the spatial resolution of such systems depends on the number of sources and detectors, this component cost increases with resolution.
An alternative optical touch screen technology, based on integrated optical waveguides, is disclosed in U.S. Pat. Nos. 6,351,260, 6,181,842 and 5,914,709, and in U.S. Patent Application Nos. 2002/0088930 and 2004/0201579, the contents of which are incorporated into this specification by way of cross-reference. The basic principle of such a device, with one particular waveguide layout, is shown schematically in FIG. 1. In this optical touch screen sensor design, integrated optical waveguides 10 conduct light from a single optical source 11 to in-plane lenses 12 that launch an array of light beams 13 across an input area 14. The light is collected by a second set of in-plane lenses 15 and integrated optical waveguides 16 at the other side of the input area, and conducted to a position-sensitive (ie. multi-element) detector 17. A touch event (eg. by a finger or stylus) cuts one or more of the beams of light and is detected as a shadow, with position determined from the particular beam(s) blocked by the touching object. That is, the position of any physical blockage can be identified in one dimension by the presence of a black spot, enabling user feedback to be entered into the device. Preferably, the device also includes external vertical collimating lenses (VCLs, not shown in FIG. 1) adjacent to the in-plane lenses 12, 15 on both sides of the input area 14, to collimate the light beams 13 in the direction perpendicular to the plane of the input area. In a variation disclosed in U.S. Pat. No. 7,099,553, the contents of which are incorporated herein by way of cross-reference, the array of integrated optical waveguides 10 on the transmit side may be replaced by a single optical waveguide in the form of a light pipe with a plurality of reflective facets.
The touch screen sensors are usually two dimensional and rectangular, with two arrays (X, Y) of transmit waveguides along adjacent sides of the screen, and two corresponding arrays of receive waveguides along the other two sides of the screen. As part of the transmit side, in one embodiment a single optical source 11 (such as an LED or a vertical cavity surface emitting laser (VCSEL)) launches light (eg. via a 1×N tree splitter 18) into a plurality of waveguides 10 that form both the X and Y transmit arrays. The X and Y transmit waveguides are usually arranged on an L shaped substrate 19, and likewise for the X and Y receive waveguides, so that a single source and a single position-sensitive detector can be used to cover both X and Y dimensions. However in alternative embodiments, a separate source and/or detector may be used for each of the X and Y dimensions. On each side the arrays of waveguides and lenses are positioned within the bezel of the screen, and to minimise the width of the bezel, it is desirable for the transmit and receive arrays to be as narrow as possible. This is especially important for small devices such as mobile phones.
A further advantage of this configuration is that all beams are detected simultaneously, enabling much taster scanning speeds compared to more conventional configurations with arrays of paired sources and detectors, where the sources and/or detectors are sequentially activated to determine if any source/detector pathways are blocked. In this waveguide-based configuration, spatial resolution can be increased simply by adding more waveguides, without affecting the source/detector component cost or the scanning speed.
Key components of waveguide-based optical touch screen sensors are the optical waveguides themselves which, as disclosed in U.S. Pat. No. 5,914,709, are in the form of arrays of waveguides integrated onto a substrate. Such integrated optical waveguides are well known in the art, and typically consist of a patterned, light guiding core layer (of refractive index n1) surrounded by a cladding material (of refractive index n2, where n2<n1) and mounted on a mechanically robust substrate. Light propagating along each waveguide is guided within the core by the refractive index difference between core and cladding. The core region is generally elongated in the propagation direction, is square or rectangular in cross section, and is usually surrounded by cladding material, which can be considered to consist of a lower cladding (in contact with the bottom face of the core) and an upper cladding (in contact with the other three faces). However this need not necessarily be the case, and as disclosed in U.S. patent application No. US 2005/0089298A1, incorporated herein by reference in its entirety, there are some situations where it is advantageous for at least one portion of the core to be flee of contact with cladding material on at least one face. Also, the lower cladding may be omitted if the substrate material has the appropriate transparency and refractive index.
To be suitable for use in consumer electronics devices incorporating touch screen sensors, optical waveguides and the materials and processes used to fabricate them have to satisfy a number of requirements. Obviously they need to be transparent at the operating wavelength, typically in the near infrared near 850 nm. With this as a given, the materials must firstly be competitive in price, ie. amenable to an inexpensive fabrication process. Secondly, they must be reliable, with a high degree of resistance to environmental challenges such as mechanical stress (bending, crushing and dropping of devices), thermal stress (extremes of temperature and rapid temperature change) and chemical stability (water and other liquids, vapours and ambient ultraviolet light). Thirdly, they must be compatible with the touch screen assembly as a whole.
Integrated optical waveguides have typically been produced from rigid materials such as silicate glass, on rigid substrates such as silicon wafers or glass wafers, and fabricated using semiconductor style processing techniques, namely, chemical vapour deposition, vacuum deposition, photolithography, reactive ion etching and the like. However these techniques require expensive capital equipment, and for reasons of cost and ease of fabrication, it is highly preferred to form the waveguides (and associated lenses) out of a photo-patternable polymer material. The cost advantage of polymers can be improved upon if the waveguides can be fabricated via a photolithography/wet etch process or a moulding process, because the capital cost of setting up a fabrication plant is much lower than for other waveguide patterning techniques such as reactive ion etching (RIE). U.S. Pat. No. 5,914,709 suggests the photo-patternable polymer benzocyclobutene (BCB) as the waveguide material, but does not consider the properties that a waveguide material must have to be suitable for use in touch screen sensors.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.